Noise is a significant concern in many communication systems and devices, including mobile phones; personal radios; wired or wireless headsets; in-the-ear (ITE) headsets; hearing protectors; and hearing aid devices. There are many sources of noise, including different types of environmental sounds and ambient noises; as well as noises generated within the hardware circuitries, particularly digital hardware circuits utilising CODECs, (Coder Decoder) ADCs (Analog to Digital Converter), DACs (Digital to Analog Converter), and DSPs (Digital Signal Processors) for audio signal processing and speech enhancement. In all these cases, the noise is accumulated at various stages of the communication system and is added to the original speech. The received final signal to the end-user therefore consists of the combined original speech and all different noise increments added during transmission.
In the case of noise generated in the hardware devices such as CODECs and/or DSPs, the circuit noise is present regardless of any external or ambient sounds or noises. This type of noise becomes very prominent when there's no ambient sound present, such as in a quiet room. The circuit noise level is directly proportional to the gain (amplification) applied in the device. Such amplification exacerbates the discomfort and annoyance to the user and can cause significant listening strain and fatigue. It can also adversely affect the quality of incoming speech. Typical examples of devices suffering from such inherent hardware noises are: call centre headsets using a digital amplifier, digital In-The-Ear (ITE) headsets/protectors and digital Hearing-Aid/Hearing-Assist devices.
As acknowledged in U.S. Pat. No. 8,126,176, in a conventional hearing aid, inputted sound is amplified without making any distinction between speech and sounds other than speech. The amplified sound is outputted to the person wearing the hearing aid. Accordingly, when environmental noise other than speech becomes loud, this may become uncomfortable for the person wearing the hearing aid. A technique has been proposed in which noise is suppressed by spectrum subtraction (SS), and the amplification ratio is varied according to the ratio between the signal power in a non-speech segment and the signal power of the inputted sound. Spectrum subtraction is a noise suppression processing method in which just the noise component is subtracted from a digital input signal by statistical estimation of the noise level of a non-speech segment.
In U.S. Pat. No. 8,126,176, a hearing aid is shown in which noise suppression processing and nonlinear compression processing are combined so that speech can be clearly heard. The hearing aid has a microphone for producing an input signal from input sound, a noise suppressor for estimating the noise component strength included in the input signal on the basis of the signal strength for each of a plurality of frequency bands in the input signal. For each of the plurality of frequency bands a noise suppression gain for suppressing the noise component strength is calculated, an adjustment amount is calculated on the basis of the signal strength and the noise component strength. The adjustment amount from the noise suppression part is used by the nonlinear compressor for calculating a reference gain on the basis of the signal strength and the specific reference gain, and adjusting the reference gain on the basis of the adjustment amount. Thus for each of the plurality of frequency bands a nonlinear compression gain for nonlinearly compressing and amplifying the input signal is calculated. A receiver reproduces an output sound from the output signal.